The present invention relates to the making of hollow shaped articles from synthetic thermoplastic material, and more particularly to improvements in the making of bottles, vials, cans analogous containers or other types of hollow articles by the blow molding method. Still more particularly, the invention relates to improvement in a multi-stage blow molding method of making hollow shaped articles from synthetic thermoplastic material.
Blow molding involves the conversion of tubular or strip-shaped parisons into hollow shaped articles by confining a parison in the cavity of an open-and-shut mold, introducing into the parison a compressed gaseous blowing medium (or withdrawing air from the cavity around the parison) to thus cause the parison to expand and to conform its exterior to the outline of the surface surrounding the mold cavity, and allowing the expanded body to cool sufficiently to retain its shape and to reduce the likelihood of adherence to parts of the machine and/or to other shaped bodies upon removal from the cavity.
It is further known to resort to a multi-stage blowing method. This involves the conversion of a parison into a hollow blank which may but need not closely resemble the ultimate product, and to thereupon convert the blank into a shaped article. The conversion of parisons into blanks takes place in a first step and the conversion of blanks into shaped articles takes place in a further step. The parisons are preferably obtained by extruding a continuous tube or web through the nozzle of a suitable extruding machine and severing the tube or web at regular intervals so that the tube or web yields a succession of tubular or strip-shaped parisons.
The conversion of parisons into blanks and/or the conversion of parisons or blanks into shaped articles can be effected by resorting to a blowing mandrel or a blowing needle. An advantage of the mandrel is that it can be used as a means for centering a hollow article or blank in the respective mold cavity as well as a means for shaping and calibrating the neck of a bottle or an analogous container. The needle is used if the parison or blank is sealed at both ends. During travel from the blowing station to the station where the parisons are converted into blanks, from the blowing station to the station where the parisons are immediately converted into shaped articles, or from the station for the making of blanks to the station for the making of shaped articles, the parisons or blanks can move along straight, arcuate, horizontal, vertical or otherwise configurated and/or oriented paths.
The length of a complete cycle of making shaped articles directly from parisons involves a movement of the mold from the extruding station to the blowing station, expanding the parison at the blowing station, cooling and thus obtained shaped article, and return movement of the mold to the extruding station. This length is determined mainly by three factors, i.e., the duration of the interval which is required for the extrusion of a parison, the duration of the interval which is required to expand the parison at the blowing station, and particularly the duration of the interval for cooling of the shaped article before the mold is allowed to open in order to eject the finished product from the cavity. The interval of extrusion in a modern extruding machine for tubular or otherwise shaped parisons is extremely short so that it does not exert an appreciable influence on the overall length of a complete cycle. The interval of expansion of a parison in the mold cavity is also very short; in fact, one can speak of practically instantaneous conversion of a parison into a shaped article. The interval of transfer of parisons from the extruding to the blowing station is also very short, especially since the parisons are normally obtained by severing a continuously extruded tube or strip at regular intervals so that the mold whose cavity has received a freshly severed parison must be removed from the extruding station as rapidly as possible in order not to interfere with the extrusion of the next-following parison.
The longest interval is always the one which is required to allow for satisfactory cooling and rigidifying of the shaped article before the article can be removed from the mold cavity. The duration of this interval depends, of course, on the size of the shaped article, on the thickness of its walls, on the nature of thermoplastic material, and on the intensity of the cooling action. The cooling of those portions which contain relatively large quantities of material takes much longer than the cooling of relatively thin walls or the like. For example, the cooling of the neck portion of a plastic bottle takes much longer than the cooling of the main body portion. Satisfactory cooling and hardening of the shaped article is desirable for many reasons which are in part obvious, i.e., to reduce the likelihood of deformation during transport from the blowing station and/or during further processing, to avoid contamination of the machine with plasticized material of the articles, and to prevent the articles from adhering to each other, to the conveyors and/or to stationary parts of the machine.
It was already proposed to produce hollow blow molded articles in several stages, not only when the articles are expanded exclusively by resorting to a gaseous fluid but also when the articles are produced in accordance with the so-called biaxial stretching procedure as disclosed, for example, in U.S. Pat. No. 2,919,462 to Friden. In each instance, the parison is converted first into a hollow blank which may but need not resemble the final product, and the blank is thereupon converted in a mold into a shaped article or final product. German Auslegeschrift No. 1,084,908 discloses an apparatus wherein the blowing mandrel extends into a nest and is surrounded therein by a freshly extruded parison which is closed at one end and is immediately caused to assume the shape of a hollow blank. The mandrel is then transferred to a blowing station while the blank is suspended thereon, and the blank is finally expanded in the cavity of a blow mold.
The biaxial stretching can be carried out in a similar apparatus. The main difference is that the latter apparatus employs a discrete first mold for the shaping of blanks and a discrete second mold for the shaping of final products. In each instance, the longest interval in the making of a blank or final product is the one which involves cooling and hardening of the body. Almost invariably, the cooling of the final product takes up more time than the cooling of blanks. This necessitates certain adjustments which often affect the quality of the blanks, mainly because the blanks are cooled longer than desirable and are not in an optimum condition for conversion into shaped articles. This occurs even if the mold for the shaping of final products is forcibly cooled with water or another fluid. It will be readily understood that pronounced cooling of blanks is undesirable because their material should remain sufficiently deformable to allow for conversion into shaped articles of predetermined size and shape.